Field of the Invention
This invention relates to a process for producing cellulose triacetate photographic film base by solution casting and more particularly to a process for producing cellulose triacetate film base at increased speed and with improved clarity.
The preparation of cellulose ester film and particularly cellulose triacetate film includes the steps of forming a dope solution by dissolving cellulose triacetate in a suitable solvent such as dichloromethane. The dope has a concentration of cellulose triacetate in the solvent of from about 15 to about 35 percent by weight. The dope is filtered through one or more filtration devices to remove solid or gelatinous particles from the dope which will cause imperfections if permitted to remain in the dope during subsequent steps in the procedure. The dope is then extruded through a hopper from which it is cast onto a highly polished wheel or continuous band where the dope is "cured" by evaporation of the solvents. Finally, the cured dope is stripped from the wheel or band, dried further and wound as a film.
The rate at which cellulose triacetate film can be prepared is dependent upon the concentration of the cellulose triacetate in the dope solution and the related property of viscosity. These two characteristics, coupled with the temperature of the dope and the pressure requirements of filtration units and other apparatus in the system, control the rate at which the cellulose triacetate film can be prepared. The pressure requirements of the filter presses is a characteristic of the device which cannot be exceeded because of safety reasons. Further, as the particular filters, and especially the filters for fine particles become clogged for any reason either due to the viscosity of the dope or because of the high concentration of the dope, the flow rate (throughput) must be decreased in order to remain within the safety tolerances of the device. This also is true should finer filter mesh size be employed in the filtering devices to bring about even a higher degree of homogeneity in the dope. Thus, the rate of flow through the filtering devices can only be increased or maintained by raising the inlet pressure of the dope and this is not prudent or practical because of safety reasons. Thus, both alternatives result in decreased production capacity. Finally, an increase in dope concentration which may be desired to decrease the drying load on the wheel or continuous band, results in an increase in the dope viscosity and again an increase in the pressure within the filter housing to achieve the same flow rate through the filter. Thus, the maximum pressure rating of the filter device limits capacity by limiting the rate of flow due to buildup on the filter media, filter media size, and dope concentration. Further, as the pressure drop across the filter media increases either because of obstruction of the media or decreased opening size of the media, more frequent shut-down is required, thus further decreasing capacity.
As a result, there is a need for a process for producing cellulose triacetate film at higher capacities utilizing dope solutions of higher concentration without impairing the clarity of the film.